An automatic survey instrument that automatically tracks a target emits light including different wavelength bands from a telescope, receives the reflected light from the target, and divides the reflected light into collimation light for collimation on the target with the naked eye, range-finding light for measurement of a distance to the target, and tracking light for automatic tracking by detecting a position of the target. As a wavelength dividing means, a dichroic prism is used in many cases. The telescope includes the dichroic prism, a telescopic optical system, a range-finding light emitting optical system, a range-finding light receiving optical system, a tracking light emitting optical system, and a tracking light receiving optical system.
FIG. 4 shows, as an example of the above-described automatic survey instrument, a configuration diagram of a dichroic prism and an optical system around the dichroic system inside a telescope described in Patent Literature 1. The reference sign 20 denotes a dichroic prism, wedged prisms 22 and 23 are attached to opposite surfaces of a pentagonal prism 21, and at borders between the prisms 21 and 22 and between the prisms 21 and 23, a first dichroic mirror surface 24 and a second dichroic mirror surface 25 are respectively formed.
Reflected light from a target passes through an objective lens 5 on an optical axis O and enters through the dichroic prism 20. The first dichroic mirror surface 24 transmits wavelengths of 400 nm-650 nm of the reflected light, and reflects wavelengths of 650 nm-850 nm. The second dichroic mirror surface 25 transmits wavelengths of 720-850 nm of infrared light, and reflects wavelengths of 650-720 nm. With this dichroic prism 20, reflected light from a target is divided into collimation light L1 (400 nm-650 nm), range-finding light L2 (800 nm), and tracking light (650 nm), and the collimation light L1 is guided to a telescopic optical system such as a focusing lens 6, an erect prism 7, a focus mirror 8, and an eyepiece lens 9, the range-finding light L2 is guided to a range-finding light receiving optical system such as a range-finding light receiving sensor 10, and the tracking light L3 is guided to a tracking light receiving optical system such as a tracking light receiving sensor 11.
Here, in an actual optical system, as shown in FIG. 4, for SN ratio improvement, an optical absorption filter 27 and an optical absorption filter 28 which absorb visible light are added to range-finding light and tracking light output surfaces although these are not shown in the drawings of Patent Literature 1. This is because, although the visible light is divided by the first dichroic mirror surface 24, it is difficult to completely divide all wavelength bands of visible light.